The Atleti prospect has been scouted by United first-team coach Phil Neville (as per Richard Martin at The Telegraph) and is being followed by Arsenal and Chelsea (as per David McDonnell at The Mirror), so here is a scouting report on the 19-year-old Spanish youth international.
Saul Season Summary
He and Oliver Torres are two Atleti teenage midfielders striving to emulate Koke's rise from the cantera to the starting XI of Diego Simeone's La Liga-contending Atleti side.
The loan to Rayo gives Saul the edge in starting 15 more La Liga games than Oliver, a role player in Atleti's world-class squad.
You are still waiting to see more from Oliver whereas Saul's displays for Rayo have been so deplorable that you worry his career will mirror that of his brother, Aaron Niguez—elite among youngsters but substandard at top-flight level.
Saul has been Tom Cleverley-like in midfield and it gets worse when he starts at centre-back.
Well, Saul's passing is erratic: some days, he completes 92 percent of his passes, other days, he is an ideal hoof-ball defender to play under Crystal Palace manager Tony Pulis.
Rayo concede a staggering 2.7 goals per game due to players such as Saul exhibiting awful positional awareness, limited defensive nous and an inability to remain level-headed in the face of adversity.
Playing at centre-back, Saul zoned out in the 1-0 defeat to Valencia.
He inadvertently passed the ball to Valencia centre-forward Jonas which led to Rayo conceding.
In the 3-1 defeat to Osasuna, Saul, starting at centre-back, inexplicably passed the ball to Osasuna midfielder Raoul Loe.
Loe set up his teammate Roberto Torres who finished confidently past Rayo goalkeeper Ruben Martinez.
Saul's tackling efficiency has plummeted.
When he first started playing for Rayo, he completed 89 percent of his tackles, now it is 49, which is inexcusable for a centre-back, let alone a midfielder.
His tackling style is hope and lunge, as evident in Rayo's 1-0 win over Real Sociedad.
He was turned inside-out by Sociedad centre-forward Imanol Agirretxe, who averages 0.3 dribbles per game.
Saul would get slaughtered trying to keep up with Liverpool's Luis Suarez, who averages 2.7 DPG more than Agirretxe.
Saul's lack of emotional maturity was on display in a 1-0 win over Almeria.
He impeded the run of Almeria's Liverpool loanee Suso.
Now here is why Saul's choice to commit a professional foul was Brick Tamland-stupid: Rayo had reinforcement at the back, Suso would have had to mimic Lionel Messi to score, the game was in the balance at 0-0 and Saul was already on a yellow card.
Referee Delgado Ferreiro dismissed Saul and he proceeded to lose the plot.
He begged referee Ferreiro to overturn the decision, saying it was his first not second yellow card. Saul would later rue his brain fade as the Spanish Football Federation extended Saul's one-match ban to three for attempting to deceive referee Ferreiro (as per Football Espana).
After Saul's plea fell on deaf ears, he was set to square up to Almeria centre-back Hernan Pellerano, only for Rayo captain Roberto Trashorras and teammate Alberto Perea to intervene.
Saul For United?
When he bottoms out in performance, the only way is up, so invariably he is bound to improve sooner or later.
Is he United quality? He is not even Sunderland quality at this present moment.
If you hate Cleverley, then Saul will bring the worst out of you.
During the 2012 UEFA European Under-19 Championship, he was rated in the same class as Pogba.
Former FIFA World Cup champion, FIFA Player of the Year and Ballon d'Or recipient Zinedine Zidane, who is currently on the Real Madrid coaching staff, is the latest footballing figure to remind Sir Alex Ferguson of his monumental blunder in letting Pogba leave for Juventus, as per Simon Mullock at The Mirror:
The word 'mistake' is not something you would normally associate with [Sir] Alex Ferguson's time at Manchester United.
But I do think it was a mistake letting Paul Pogba leave.
He will go on to be one of the best central midfield players in Europe and he was the natural successor to Paul Scholes, someone they could have built the team around.
He is with Juventus and doing very well but, as always, Real Madrid will be looking to add the best players in the world.
It will not be a mistake if United pass up on signing Saul.
He will need to do an "Aaron Ramsey" (incalculable exponential improvement) and get lucky with injuries to his teammates to break into Simeone's Atleti starting XI next season.